Tear-inspired material can be tuned to repel or attract water

Last year, a group of Harvard University scientists led by Dr. Joanna Aizenberg announced the development of a highly-hydrophobic (water-repellant) material known as SLIPS, or Slippery Liquid Porous Surfaces. The material is remarkable, in that it repels virtually any liquid. Now, Aizenberg and colleagues have created a new material inspired by human tears, the repellency of which can be fine-tuned for different applications.

Like SLIPS, the new material consists of a substrate infused with a continuous liquid film – just like the human eye is covered with a film of tears. Whereas SLIPS has a rigid substrate, however, the substrate of the new material is elastic.

In the material’s relaxed state, the liquid sits on top of the substrate, its smooth surface keeping other liquids from sticking to it. When the substrate is stretched (or poked, or made to swell), however, its pores get larger, causing the surface of the overlying film to become rougher and less hydrophobic.

Exactly how much less hydrophobic it becomes can be tweaked by varying the amount that it’s being deformed. A droplet of liquid rolling down its surface can even be stopped in its tracks by stretching the material, as is illustrated in the following video.

Although it’s not the first material to demonstrate “switchable wettability,” it is reportedly the first that doesn’t simply switch between being entirely hydrophobic and entirely hydrophilic (water-absorbing). Its change in state could be brought about not just through mechanical stretching, but also via changes in environmental parameters such as temperature, light, chemical signals, or magnetic or electric fields.

Additionally, the new material is transparent when relaxed, but becomes increasing opaque as it’s being stretched. According to Harvard, this quality could perhaps lead to its use in tents that block sunlight when dry, but repel water and let in more light when it becomes cloudy and rainy.

Other applications could include self-cleaning optics, textiles, building materials, and pipeline linings that optimize the rate of flow depending on fluid volume and other factors.

A paper on the research was published this week in the journal Nature Materials. Another example of the material's properties can be seen below.